1. Technical Field
The present invention relates to a semiconductor light emitting device equipped with a light emitting region that is formed on a GaAs substrate utilizing a GaAlAs system material, and a manufacturing method therefor. The present invention particularly relates to the device where an operation life test is preferable under high temperature and high moisture (for example, 85.degree. C. and 85%) and at a high output of infrared rays and visible rays. Moreover, it particularly relates to the device and the manufacturing method therefor where a yield for a pellet is desirably high.
2. Background Art
FIG. 1A is a structure according to the conventional semiconductor light emitting device.
In the same figure, by use of a slow cooling method, on a p-type GaAs substrate, there are sequentially grown thereon a Zn-added p-type GaAlAs thick film clad layer 13, a Zn-added p-type GaAlAs active layer 12 (where an emitted light wavelength is a mixed crystal ratio for the infrared ray), and an Te-added n-type GaAlAs clad layer 11 so that a double hereto structure is obtained. In order to enable a high output thereof, the structure is generally such that the p-type GaAs substrate serving as a light-absorbing layer is removed and not used.
In order to realize a sufficient efficiency for the double hetero structure, an energy gap (.DELTA.eV) between the p-type GaAlAs thick film clad layer 13 and the n-type GaAlAs clad layer is set to more than 0.3 eV. However, due to the slow cooling method employed, an AlAs mixed crystal ratio for the n-type GaAlAs clad layer 11 is set to 0.7 or greater where the mixed crystal ratio therefor is little affected by temperature change. The higher AlAs mixed crystal ratio is, the less temperature dependency therefor is. This is because the mixed crystal ratio fluctuates less in a growth direction in this case. Thereby, a stress within a grown wafer is made small so as to increase a pellet-processing yield. The pellet-processing yield is represented by the number of pellets obtained from a single wafer.
The term "AlAs mixed crystal ratio" means an AlAs mixed crystal ratio relative to a GaAs crystal. For example, an AlAs mixed crystal ratio 0.7 means a composition of Ga(0.3).Al(0.7).As.
On the other hand, in an element structure of the conventional semiconductor light emitting device there is a problem where a crack occurs in the GaAlAs substrate in the course of a pellet-making process, so that the yield therefor is reduced. The crack in the GaAlAs substrate accounts for a stress generated in a grown layer due to a difference in the AlAs mixed crystal ratio (the mixed crystal ration of p-type GaAlAs active layer 12 excluded). Such a stress causes a bow (sori) in the GaAlAs substrate so as to generate the crack. Therefore, the reduction in the yield is prevented in a manner that the mixed crystal ratio (X.sub.A1) between the p-type GaAlAs thick film clad layer 13 and the n-type GaAlAs clad layer 11 is made equal or the mixed crystal ratio for n-type GaAlAs clad 11 is made higher than layer 13. FIG. 1B is a graph showing distribution of the AlAs mixed crystal ratio along a direction of growth.
By such a composition, an operation life test using 50 mA results under the high temperature and moisture of 85.degree. C. and 85%, respectively:
Al.sup.+ +30H.sup.- .fwdarw.Al(OH).sub.3 .fwdarw.1/2Al.sub.2 O.sub.3 +3/2H.sub.2 PA1 Ga.sup.+ +30H.sup.- .fwdarw.Ga(OH).sub.3 .fwdarw.1/2Ga.sub.2 O.sub.3 +3/2H.sub.2
Referring to FIG. 2, the above deterioration mechanism is presumably carried out, so that there is formed a light-absorbing layer on the surface of the pellet and external output therefrom is significantly reduced. In other words, a constant forward current causes Al to react with OH.sup.- ion. Then, on the surface of the pellet there is formed an oxide layer such as Al.sub.2 O.sub.3 and Ga.sub.2 O.sub.3 serving as the light-absorbing layer. Thereby, the deterioration in efficiency for the light emitting element (device) occurs. However, without presence of the constant forward current under the high temperature and moisture such as in a condition of 85.degree. C. and 85% and the device being left alone for 2000 hours, the oxide layer on the surface of the pellet is not detected and a decline of the external output due to the deterioration is not observed. Therefore, it seems that a radiation ray from a junction also contributes to the forming of GaAlAs layer.
Accordingly, in the conventional semiconductor light emitting device there is caused a problem where there is formed the light-absorbing layer on the surface of the pellet under the high temperature and moisture so as to significantly reduce the external output. As a result thereof, an environmental condition under which the device can be utilized is undesirably limited, so that the device can only be usable in the vicinity of a room temperature.